This invention relates to a loopback command arrangement for use either in a line terminating station of a PCM (pulse code modulation) communication network or system or in an intermediate repeater station thereof and to a loopback command system for use in a PCM communication network.
Ordinarily, a PCM communication network comprises a first and a second transmission route and a first line terminating station, a plurality of intermediate repeater stations, and a second line terminating station successively along the first and the second transmission routes. In other words, the PCM communication network is a two-way communication network. The first and the second line terminating stations are usually similar in structure. When attention is directed to one of the first and the second line terminating stations, the other is a counterpart line terminating station. It is ordinary that each intermediate repeater station is a nonattendant station.
By way of example, the first line terminating station comprises a frame processing arrangement for multiplexing input signals and transmission overhead or housekeeping bits or data into successive frames of a first PCM (pulse code modulated) signal for delivery or transmission to the first transmission route. The transmission overhead bits include bits which carry a frame alignment or synchronizing signal. The frame processing arrangement demultiplexes successive frames of a second PCM signal received from the second transmission route into output signals and reception overhead bits. In an ordinary state or mode of operation of the PCM communication network, the output signals are reproductions of input signals which are multiplexed in the second PCM signal at the second line terminating station. The reception overhead bits are reproductions of transmission overhead bits which are likewise multiplexed. In the manner known in the art, the reception overhead bits need not be produced from the frame processing arrangement.
A loopback command system is useful in such a PCM communication network in locating a trouble that may take place in one of the first and the second transmission routes and the counterpart line terminating and the intermediate repeater stations. Various loopback command systems are already known.
Briefly according to U.S. Pat. No. 3,692,964 issued to Roberto Camiciottoli et al and assigned to Societa Italiana Telecommunicazioni S. p. A., a loopback command system comprises in the first line terminating station a pulse generating arrangement for supplying the first transmission route with a sequence of iteratively selected ones of different identification codes assigned to the respective intermediate repeater stations. Each intermediate repeater station comprises a detector for detecting the identification code assigned thereto to produce a detection signal and a switch responsive to the detection signal for looping the code sequence back to the first line terminating station.
Roughly according to U.S. Pat. No. 4,319,080 issued to Kiyoshi Kuwahara and assigned to Anritsu Electric Company Limited of Tokyo, Japan, supervisory frequencies are allotted to the intermediate repeater and the second line terminating stations, respectively. The first line terminating station supplies the first transmission route with a test signal consisting of those successively generated pulse train groups each of which contains a frequency component equal to one of the supervisory frequencies. Each of the intermediate repeater and the second line terminating stations comprises a band-pass filter for the supervisory frequency assigned thereto for looping the test signal back to the first line terminating station.
In such conventional loopback command systems, it is necessary on testing the trouble to make the first line terminating station suspend delivery of the first PCM signal and send instead a test signal. The test signal may be the identification code sequence.